Re: Interrupt routine usage not shown by top in 8.0





--- On Thu, 3/12/09, Scott Long <scottl@xxxxxxxxxx> wrote:

From: Scott Long <scottl@xxxxxxxxxx>
Subject: Re: Interrupt routine usage not shown by top in 8.0
To: barney_cordoba@xxxxxxxxx
Cc: current@xxxxxxxxxxx
Date: Thursday, March 12, 2009, 7:42 PM
Barney Cordoba wrote:
I'm fireing 400Kpps at a udp blackhole port.
I'm getting 6000 interrupts
per second on em3:

testbox# vmstat -i; sleep 1; vmstat -i
interrupt total rate
irq1: atkbd0 1 0
irq6: fdc0 1 0
irq17: uhci1+ 2226 9
irq18: uhci2 ehci+ 9 0
cpu0: timer 470507 1993
irq256: em0 665 2
irq259: em3 1027684 4354
cpu1: timer 470272 1992
cpu3: timer 470273 1992
cpu2: timer 470273 1992
Total 2911911 12338

interrupt total rate
irq1: atkbd0 1 0
irq6: fdc0 1 0
irq17: uhci1+ 2226 9
irq18: uhci2 ehci+ 9 0
cpu0: timer 472513 1993
irq256: em0 668 2
irq259: em3 1033703 4361
cpu1: timer 472278 1992
cpu3: timer 472279 1992
cpu2: timer 472279 1992
Total 2925957 12345


top -SH shows:

PID STATE C TIME CPU COMMAND
10 CPU3 3 7:32 100.00% idle
10 CPU2 2 7:32 100.00% idle
10 RUN 0 7:31 100.00% idle
10 CPU1 1 7:31 100.00% idle

This implies that CPU usage is substantially
under-reported in general
by the system. Note that I've modified
em_irq_fast() to call em_handle_rxtx() directly rather than
scheduling a task to illustrate
the problem


With unmodified code, what do you see? Are you sending
valid UDP frames with valid checksums and a valid port, or
is everything that you're blasting at the interface
getting dropped right away? Calling em_handle_rxtx()
directly will cause a very quick panic once you start
handling real traffic and you encounter a lock.

Scott

I think you're mistaken. I'm also accessing the system via an em port
(and running top) and em_handle_rxtx() is self contained lock-wise.
The taskqueue doesn't obtain a lock before calling the routine.

As I mentioned, they're being dumped into a udp blackhole, which implies
that I have udp.blackhole set and the port is unused. I can see the
packets hit the udp socket so its working as expected:

853967872 dropped due to no socket

With unmodified code, the tasq shows 25% usage or so.

I'm not sure what the point of your criticism for what clearly is a test.
Are you implying that the system can receive 400K pps with 6000 ints/sec
and record 0% usage because of a coding imperfection? Or are you implying
that the 25% usage is all due to launching tasks unnecessarily and process
switching?

Barney



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